sched: Remove unnecessary iteration over sched domains to update nr_busy_cpus

nr_busy_cpus parameter is used by nohz_kick_needed() to find out the
number of busy cpus in a sched domain which has SD_SHARE_PKG_RESOURCES
flag set.  Therefore instead of updating nr_busy_cpus at every level
of sched domain, since it is irrelevant, we can update this parameter
only at the parent domain of the sd which has this flag set. Introduce
a per-cpu parameter sd_busy which represents this parent domain.

In nohz_kick_needed() we directly query the nr_busy_cpus parameter
associated with the groups of sd_busy.

By associating sd_busy with the highest domain which has
SD_SHARE_PKG_RESOURCES flag set, we cover all lower level domains
which could have this flag set and trigger nohz_idle_balancing if any
of the levels have more than one busy cpu.

sd_busy is irrelevant for asymmetric load balancing. However sd_asym
has been introduced to represent the highest sched domain which has
SD_ASYM_PACKING flag set so that it can be queried directly when
required.

While we are at it, we might as well change the nohz_idle parameter to
be updated at the sd_busy domain level alone and not the base domain
level of a CPU.  This will unify the concept of busy cpus at just one
level of sched domain where it is currently used.

Signed-off-by: Preeti U Murthy<preeti@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: svaidy@linux.vnet.ibm.com
Cc: vincent.guittot@linaro.org
Cc: bitbucket@online.de
Cc: benh@kernel.crashing.org
Cc: anton@samba.org
Cc: Morten.Rasmussen@arm.com
Cc: pjt@google.com
Cc: peterz@infradead.org
Cc: mikey@neuling.org
Link: http://lkml.kernel.org/r/20131030031252.23426.4417.stgit@preeti.in.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 20 insertions, 18 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 074551a792f7..df77c605c7a6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6534,16 +6534,16 @@ static inline void nohz_balance_exit_idle(int cpu)
 static inline void set_cpu_sd_state_busy(void)
 {
         struct sched_domain *sd;
+        int cpu = smp_processor_id();
 
         rcu_read_lock();
-        sd = rcu_dereference_check_sched_domain(this_rq()->sd);
+        sd = rcu_dereference(per_cpu(sd_busy, cpu));
 
         if (!sd || !sd->nohz_idle)
                 goto unlock;
         sd->nohz_idle = 0;
 
-        for (; sd; sd = sd->parent)
-                atomic_inc(&sd->groups->sgp->nr_busy_cpus);
+        atomic_inc(&sd->groups->sgp->nr_busy_cpus);
 unlock:
         rcu_read_unlock();
 }
@@ -6551,16 +6551,16 @@ unlock:
 void set_cpu_sd_state_idle(void)
 {
         struct sched_domain *sd;
+        int cpu = smp_processor_id();
 
         rcu_read_lock();
-        sd = rcu_dereference_check_sched_domain(this_rq()->sd);
+        sd = rcu_dereference(per_cpu(sd_busy, cpu));
 
         if (!sd || sd->nohz_idle)
                 goto unlock;
         sd->nohz_idle = 1;
 
-        for (; sd; sd = sd->parent)
-                atomic_dec(&sd->groups->sgp->nr_busy_cpus);
+        atomic_dec(&sd->groups->sgp->nr_busy_cpus);
 unlock:
         rcu_read_unlock();
 }
@@ -6767,6 +6767,8 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
 {
         unsigned long now = jiffies;
         struct sched_domain *sd;
+        struct sched_group_power *sgp;
+        int nr_busy;
 
         if (unlikely(idle_cpu(cpu)))
                 return 0;
@@ -6792,22 +6794,22 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
                 goto need_kick;
 
         rcu_read_lock();
-        for_each_domain(cpu, sd) {
-                struct sched_group *sg = sd->groups;
-                struct sched_group_power *sgp = sg->sgp;
-                int nr_busy = atomic_read(&sgp->nr_busy_cpus);
+        sd = rcu_dereference(per_cpu(sd_busy, cpu));
 
-                if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
-                        goto need_kick_unlock;
+        if (sd) {
+                sgp = sd->groups->sgp;
+                nr_busy = atomic_read(&sgp->nr_busy_cpus);
 
-                if (sd->flags & SD_ASYM_PACKING
-                    && (cpumask_first_and(nohz.idle_cpus_mask,
-                                          sched_domain_span(sd)) < cpu))
+                if (nr_busy > 1)
                         goto need_kick_unlock;
-
-                if (!(sd->flags & (SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING)))
-                        break;
         }
+
+        sd = rcu_dereference(per_cpu(sd_asym, cpu));
+
+        if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
+                                  sched_domain_span(sd)) < cpu))
+                goto need_kick_unlock;
+
         rcu_read_unlock();
         return 0;